INSERT where not exists with foreign key

My Google foo isn't working on this question, probably because I don't
understand the question well enough. I'm using Postgres 11.3 on Centos 7.
I'm trying to insert a record in table A with a foreign key to table B, but
only where there is not already a foreign key in A to B. So assume this
simple structure:

Table A
A.key Integer
A.something text
A.Bkey Integer [foreign key to table B, column B.key

Table B (or View C)
B.key Integer
[more columns]

Thinking that it might simplify matters, I created a view to table B that
only includes records with no foreign key in Table A. But still, after
reading the documentation and Googling, I can't work out what seems like it
should be simple. Among the many things that didn't work is:

INSERT INTO A(something,A.Bkey)

VALUES ('text',
(SELECT C.key FROM C)

But this didn't work because the subquery returned more than one value. Of
course I want it to return all values, but just one per insert.

I can do this outside of Postgres, but would like to learn how to do this
with SQL.

Chuck Martin
Avondale Software